Known as Hellflame in the Enix of America localization, the skill is learned by reaching rank 5 of the Druid vocation and will deal 210~250 damage to all enemies. Unlike other breath attacks, the skill costs 20 MP to use, presumably to prevent it from being abused during battle.
Hellfire breath is a powerful Fire-elemental breath, used by high level enemies. Most Legacy Bosses learn this skill at some point. Unlike it's previous static range, Hellfire's damage is now dependent on the boss' level, calculated as (70+(lvl^2/22))*0.9~1.1). In other words, the base damage of the skill is 162~198, and the maximum at level 99 is 463~567.